Over the past several months, they have had several B266 reed relay cards fail with shorted outputs. As they had been sent to us for repair, we had replaced the shorted internal relays, tested them and shipped them back to the client. We had no idea that they were from the same system and the same channel of I/O.

Immediately, we suspected that either the relays had finally reached their end of life cycle or that there was a field wiring issue. We visited the client’s site and determined quickly that there was no field issue, since the failing cards were in various slots, but all from the same channel of I/O.

We however did notice that there was a voltage drop of about 10 VAC across a CLOSED output contact! There should have been almost zero volts across these contacts.

This channel of I/O was the even channel driven by a P-421 auxiliary power supply coupled to a P-451 remote I/O power supply. We performed some basic voltage and A.C. ripple tests while the unit was still in service and all seemed OK. There was a planned outage the following day so we suggested that the P-412 power supply be replaced then as a precautionary measure.

The next day the power supply was replaced. The client’s technician then measured the voltage drop across all of the closed contacts and found that they were all near zero as they should be.

The P-421 power supply was somehow causing the issue and subsequent failures of the B266 output cards.

The power supply was sent to our shop for repair. We connected it to a fully loaded channel of I/O containing an array of B266 modules and found that there was indeed voltage drop across the closed contacts.

In the troubleshooting process, a failed component in the power supply was actually causing the outputs to cycle on and off extremely fast. This appeared to us as a voltage drop across the “closed” contacts.

The defective part was replaced, along with the units’ capacitors due to their age, and it was re-tested and then returned to the client.

A few days later, another client from Canada called with what he said was an unusual problem. He described a very similar issue, only this was on a Modicon 384 controller. I asked how the I/O was powered, and he replied that there was a P-421 power supply installed there.

I suggested that he replace it. He did so and called back stating that it had cleared the problem.

Of course it’s always great to locate and fix a clients’ problem with their equipment, but what is more fascinating in these scenarios, is that these products are the original power supplies! These units were built and installed in the late 1970’s.

No one has ever said that Modicon products were cheap or inexpensive when they purchased them, but to get almost 35 years of continuous service from them says a lot!